In a high performance multi carrier receiver the linearity and dynamic range requirements are very challenging. To meet the demanding requirements automatic gain control (AGC) can be used to adjust the dynamic range of the receiver according to the received signal level and thus relax the dynamic range of the receiver circuitry. In such a receiver an AGC control function is needed.
FIG. 1 shows a prior art AGC receiver 100 with an analogue input signal (sin.). The example of FIG. 1 shows a single carrier receiver for a telecommunication application and the input signal is an RF (Radio Frequency) signal. The input signal is modulated by the well know modulation technique called IQ-modulation. As this technique is well known to the skilled person it is not further explained here.
The communication input signal (sin) is amplified in a first amplifier 101 and then passed through a first band pass filter 102 limiting the frequency to the RF range and producing as an output a first mixer input signal (s1). This first mixer input signal is fed to a first mixer 103 outputting a first mixer output signal (s2) at an intermediate frequency, which signal is passed through a second band pass filter 104. The filter produces as an output a first attenuator input signal (s3) which is fed to a gain control unit 105 where the signal is first attenuated in a first attenuator 106. The attenuator can be replaced by an amplifier which attenuates the signal. The signal is then amplified in a second amplifier 107 and finally passed through a second mixer 108 to bring down the frequency of a gain control output signal (s4) of the gain control unit to a low frequency.
The gain control output signal (s4) is fed to a third band pass filter 109, the output of which is an analogue output signal (s5) limited to the low frequency and fed to an Analogue Digital Converter (ADC), 110. The first amplifier 101, the first band pass filter 102, the first mixer 103, the second band pass filter 104, the gain control unit 105 and the third band pass filter 109 are all parts of an analogue unit 119.
The output of the ADC is a digital input signal (s6) which is divided in two paths, an AGC input signal (s61) fed to an Automatic Gain Control (AGC) unit, 112, and an AGC compensation unit input signal (s62) fed to an AGC compensation unit 113. The AGC generates a first control signal c1 to the attenuator 106 and a second control signal (c2) to the AGC compensation unit.
The AGC compensation unit input signal (s62) is fed to the AGC compensation unit 113 which introduces an increase or decrease of a gain to the AGC compensation unit input signal (s62) based on the information in the second control signal (c2). The AGC compensation unit output signal (s7) is divided in two paths, a first path (s71) and a second path (s72), the first path being connected to a first mixer stage 114 and the second path being connected to second mixer stage 115.
A first mixer stage output signal (s81) from the first mixer stage is fed to a first channel filter (RRC), 116, and a second mixer stage output signal (s82) from the second mixer stage is fed to a second channel filter (RRC), 117.
RRC stands for Root Raised Cosine and defines the type of filter used. The RRC filters and the digital mixer stages are part of a well known IQ-demodulator producing the In-phase part (I) of the modulating signal from the first RRC filter and the Quadrature part (Q) from the second RRC filter.
IQ modulation is used to conserve bandwidth for a given data rate. This is accomplished by modulating two orthogonal data streams onto a common carrier. If the phases and amplitudes of both data stream (in-phase “I” and quadrature “Q”), then one of the sidebands is completely cancelled out. If there is no DC bias feed-through, then the carrier itself is completely cancelled out.
There are two mixer stages and two RRC filters for each carrier wave. In this example there was only one carrier wave and thus two mixer stages and two RRC filters. For a three carrier wave signal six mixer stages and six RRC filters are required. A common AGC compensation unit is feeding all mixer stages. The components; AGC compensation unit, AGC control unit, mixer stages and RRC filters are all parts of a digital unit 118 of the receiver.
The AGC unit 212 according to prior art is shown in FIG. 2. The AGC unit consists of a high, 201, and a low, 202, threshold detector which controls the AGC level up or down. The AGC level unit, 203, controls the analogue attenuation in the receiver and the AGC compensation in the digital unit. The AGC input signal (s61) is a path of the digital input signal (s6) from the ADC. The AGC input signal is divided in two paths, one fed to the high threshold detector and one to the low threshold detector. When the high threshold is detected the high threshold detector sends a third control signal (c3) to the AGC level unit and when a low threshold is detected the low threshold detector sends a fourth control signal (c4) to the AGC level unit.
The AGC level unit produces the two control signals (c1) and (c2). When the high threshold is detected the first control signal c1 informs the first attenuator 106 to increase attenuation and the second control signal c2 informs the AGC compensation unit to increase gain in the digital part to compensate for the attenuation in the analogue part. This means that the attenuation in the analogue part “AGC attenuation” is compensated with the same amount of gain increase in the digital part “AGC compensation” and the level of the first attenuator input signal (s3) will be equal to the level of the AGC compensation unit output signal (s7). When the low threshold is detected the first control signal (c1) informs the first attenuator 106 to decrease attenuation and the second control signal (c2) informs the AGC compensation unit to decrease gain to compensate for the decreased attenuation in the analogue part.
In prior art WCDMA receivers, the down-converted and RRC pulse shaped signal is normally level adjusted to a constant average level by a DAGC (Digital Automatic Gain Control) 120. The IQ-demodulator produces the In-phase part (I) of the modulating signal from the first RRC filter 116 and the Quadrature part (Q) from the second RRC filter 117. In the DAGC the received signal (I and Q) is multiplied 124 by the inverse of the averaged RMS (Root Means Square) level (X) to create a constant average level. The average level (X) is calculated in the calculator 121 on the basis of the modulated signals part (I and Q).
In particular, the modulated signal part (I) is mixed in a third mixer stage 122 with the inverse of the averaged RMS level, and a third mixer stage output signal (S91) is fed to the base band. In particular, the modulated signal part (Q) is also mixed in a fourth mixer stage 123 with the inverse of the averaged RMS level, and a fourth mixer stage output signal (S92) is fed to the base band.
Since a constant average level is enabled, the dynamic range of the received signal is now reduced to minimize the number of bits transmitted to the base band. These bits in combination with the RMS average (S93) are sent to the base band. The RMS average value is sent more seldom than the received signal to minimize the signaling bandwidth. By doing this level adjustment on each individual receiver chain, signals from several diversity branches (from different receivers) can easily be combined in the base band by a RAKE receiver (type of receiver). A rake receiver is a radio receiver designed to counter the effects of multipath fading. It does this by using several “sub-receivers” called fingers, that is, several correlators each assigned to a different multipath component.
In the analog part 119 of the receiver 100 an AGC solution is often used to increase the dynamic range in the receiver, see FIG. 1. For over range interfering signals the gain in the receiver path is reduced to maintain the normal operational range of the ADC and other circuits after the attenuator 106. The gain is controlled by the attenuator 106 in the gain control unit 105 on the basis of the first control signal (c1) generated by the AGC 212, see FIG. 2.
In ideal situation, only the desired signal 304 will come out from the AGC receiver 100, see FIG. 3. The problem however is that when the gain in the receiver chain is reduced (increased attenuation), the AGC noise 301 in the receiver 100 is increased from the normal noise level 303 due to the lower gain. The interfering signal 302 will be reduced 305 by the RRC filter 117,118 but the additional AGC noise in the desired signal bandwidth will be unchanged, see FIG. 3.
For time varying interfering signals 302 the AGC noise will vary over time due to the different states of the AGC in the receiver. Now, when the received signal is despreaded in time by the RAKE receiver, the SNR (Signal to Noise Ratio) is not optimal any more and the optimal reception performance is not obtained. SNR is closely related to the concept of dynamic, where dynamic range measures the ratio between noise and the greatest un-distorted signal on a channel. SNR measures the ratio between noise and an arbitrary signal on the channel.
The received signal with time varying noise is illustrated in FIG. 4. The AGC noise over the analogue part 119 will vary over time according to the AGC settings. The desired signal 404 and the normal noise level 403 is shown. As illustrated, the AGC noise 401 depends on the attenuation in the gain control unit 105. During a first time period (T1), with a gain level (G1), the AGC noise over the analogue part 119 is at one level (N1). During a second time period (T2), with a gain level (G2), the AGC noise is at another level (N2). The AGC compensation unit 113 introduces an increase or decrease of the gain (G1/G2) to the AGC compensation unit input signal (s62) based on the information in the second control signal (c2).
There is consequently a problem with the reception performance of an AGC receiver when. When the gain in the receiver chain is reduced the AGC noise in the receiver is increased due to the lower gain. The interfering signal will be reduced by the RRC filter but the additional AGC noise in the desired signal bandwidth will be unchanged, see FIG. 3. When the received signal is despreaded in time by the RAKE receiver, the SNR is not optimal any more and the optimal reception performance is not obtained, see FIG. 4.